'cognitive dissonance'

: There is a human tendency to doubt one's purchase decision especially after havng purchased something expensive. What is this called?

The marketing types call it 'cognitive dissonance'. There is a tendency to seek out additional materials and information (the search part of the buying process that normally comes at the front end) to rejustify the decision. This also occurs when two or more inconsistent beliefs or value systems influence behavior -- i.e. eating meat or wearing furs while believing that the slaughter of animals is wrong.